This invention relates to multi-user detection of SAW RFID tags and sensors.
RFID tags are somewhat similar to bar codes in that they each have a unique identification code, but RFID tag codes can be retrieved wirelessly without relying on close line-of-sight contact. Such RFID tags are used for identification or logistic purposes such as in warehouses, manufacturing or commercial outlets. Specific measurement parameters can also be extracted from certain SAW RFID configurations to produce a passive wireless sensor capable of conveying an identification code if required along with temperature, pressure or other similar measurements back to an interrogation reader. Such uniquely identifiable sensors would be well suited for the automotive industry where a single reader located in an automobile could communicate and monitor pressure, temperature and other useful parameters.
For the simple case of a single SAW RFID tag or sensor within the effective e range of an interrogation transceiver; the following events take place:
a. An interrogation transceiver emits an interrogation signal through its single antenna.
b. The RFID tag or sensor which is typically located 1 to 2 meters away, receives the interrogation signal via its own antenna and couples the signal to the SAW""s encoded interdigital transducer (IDT).
c. The acoustic wave produced by the IDT propagates either within the same extended IDT or propagates away and is then reflected backing reflection gratings to the IDT with encoded information specific to that particular tag or sensor.
d. This encoded signal couples from the IDT to the antenna and pro agates through the air back to the single antenna of the interrogation transceiver.
e. Identification information or sensor parameters are then extracted from the returned signal and identified by a reader.
A major problem exists if there are two or more tags or sensors within the range of the interrogation transceiver. Multiple tags or sensors will be performing steps b, c and d as outlined above more or less simultaneously and causing a collision of data at the interrogation transceiver and with consequent inability of the reader to accurately decode the identification codes.
Researchers from Germany have proposed several solutions to solve the multiple access problems. One solution, A. Pohl, F. Seifert, L. Reindl, G. Scholl, T. Ostertag and W. Pietschl, xe2x80x9cRadio Signals For Saw ID Tags And Sensors In Strong Electromagnetic Interference,xe2x80x9d Proceedings 1994 IEEE Ultrasonics Symposium, pp.195-198, 1994., outlines how a 5-path RAKE receiver is used to solve the multiple access problem. Another solution was presented, G. Ostermayer I, A. Pohl, L. Reindl and F. Seifert xe2x80x9cMultiple Access to SAW Sensors Using Matched Filter Properties,xe2x80x9d Proceedings 1997 IEEE Ultrasonic Symposium, pp.339-342, 1997., where they presented a method of matched filters to solve the problem of access to more than one passive SAW sensor that is within the range of an interrogation transceiver.
These two papers by the German researchers describe multi-user detection (MUD) which depends on spectral analysis to determine individual characteristics of each tag or sensor.
U.S. Pat. No. 5,500,651 describes an arrangement in which a first interrogation signal is transmitted from the interrogation reader. This first interrogation signal having a first read range. A first response signal is then received at the reader after which a second interrogation signal is transmitted from the reader. The second interrogation signal has a second read range which is different than said first read range. The read range can be varied by varying either the amplitude or duration of the power level of the interrogation signal. A second response signal is then received at the reader. These consecutive responses are then compared to determine a correct response signal which can be displayed.
U.S. Pat. No. 6,034,603 describes a data transmission system which includes a reader transceiver and one or more semiconductor-based tag transceivers. The reader transmits an activation signal to a semiconductor-based tag. The tag selectively transmits a response signal to the reader at a specific time determined by a detected level of communications channel activity.
U.S. Pat. No. 6,150,921 describes a system for tracking mobile tags. Cell controllers with multiple antenna modules generate a carrier signal which is received by the tags. Tags shift the frequency of the carrier signal, modulate an identification code onto it, and transmit the resulting tag signal at randomized intervals.
U.S. Pat. No. 6,377,203 describes a method for simultaneously reading a serial number and/or other information from multiple colliding RF signals from RF identification tags requires minimal additional logic in the semiconductor-based tag""s processor and provides for a powerful and rapid sorting and identification scheme. This technique employs a primary communication channel and multiple secondary channels.
U.S. Pat. No. 6,392,544 describes a radio frequency identification reader which includes a plurality of antenna elements that are spaced to define active areas. A matrix switch flexibly connects the plurality of antenna elements to an exciter circuit. Independent switches are selectively switched such that an electric field is generated between at least two antenna elements whereby radio frequency identification tags in the vicinity of the two antenna elements.
U.S. Pat. No. 6,411,199 is directed generally to the field of Radio Frequency (RF) identification systems employing an electronic transponder and a transponder reader which emits an RF interrogation signal at a first frequency which powers the electronic transponder and enables it to transmit a response on a second frequency (usually a lower frequency).
The patents listed above all relate to semiconductor RFID tags or sensors.
This invention places a unique identifier on the acoustic wave produced by the encoded IDT in step c as outlined above to allow an algorithm within the interrogation transceiver to separate out the multiple signals, and actually changes the finger geometries of the IDTs to encode specific identifiers which can be used to separate out multiple tags or sensors.
This invention introduces an encoding concept that is applicable to a variety of passive surface acoustic wave (SAW) radio frequency identification (RFID) tags or sensors for MUD. Multiple signals from the tags or sensors, all responding simultaneously when interrogated by a single wireless reader, could occur when several RFID tags or sensors are spatially located very close to each other. Such multiple RFID tags or sensors would all be within the usable radiation pattern of the reader""s single antenna and produce multiple collisions when received back at the reader. This encoding scheme is embedded into the geometries of the SAW devices to be used in detecting and identifying multiple devices within the reader""s single antenna""s range. The encoding method uses a phase modulation of selected finger pairs on each SAW device which in effect places a unique identifier on the tag or sensor""s returned impulse response. An algorithm is then used to separate out each unique identifier and reconstruct the unique identification number of each tag or sensor.
According to the invention, a group of SAW RFID tags or sensors each has a transducer with interdigitated fingers, a different set of fingers in each tag or sensor being reversed in phase to cause the tag or sensor to provide a unique identifier in a returned impulse response produced by the tag or sensor when interrogated by an interrogation signal.
Each tag or sensor may have a single pair of phase reversed interdigitated fingers. Alternatively, each tag or sensor may have multiple adjacent pairs of phase reversed interdigitated fingers. Advantageously, the phase reversed interdigitated fingers do not occupy the extreme end of an elongated transducer which is closest to either reflectors or other finger arrays thereof.
The present invention also provides a method of distinguishing between responses produced by a group of SAW RFID tags or sensors having transducers with interdigitated fingers when interrogated by an interrogation signal, including phase reversing a different set of fingers in each tag or sensor to cause the tag or sensor to provide a unique identifier in the returned impulse response.
The method may also include providing an interrogation reader to receive responses from the SAW RFID tags or sensors and distinguish therebetween by vector analysis. The vector analysis may include comparing said responses with predetermined vector models for information encoded in the RFID tags or sensors. The predetermined vector models may be stored in a programmable look-up table within the interrogation reader.
The method may also include determining probable vector combinations from the compared responses. Variation within the probable vector combinations may be detected. The method may include selecting a probable phase change within the encoded information to compensate for a detected variation.
The detection of a variation and selecting a probable phase change within other RFID encoding information may be repeated to compensate for the variation detected. The detecting and selecting may be repeated until all probable vector combinations which contain the implemented device encoded information are determined.